Can annular combustors for gas turbine engine may include a combustor assembly having a central pilot burner and a plurality of pre-mix main burners disposed about the pilot burner. The pilot burner typically receives a portion of a flow of compressed air received from a compressor and mixes the pilot burner flow with fuel to form a pilot burner air and fuel mixture. The pilot burner mixture may be swirled by flow control surfaces in the pilot burner that impart circumferential motion to the axially moving pilot burner mixture. This swirled flow continues within a diverging pilot cone and this arrangement produces an expanding, helically flowing pilot mixture which is ignited and which serves to anchor the combustor flame.
The main burners may be held in place around the pilot burner and extend through a base plate that is oriented transverse to the main burners. Similar to the pilot burner, each main burner receives a respective portion of the flow of compressed air received from a compressor. Each flow of compressed air flows through its respective main burner where it is mixed with fuel to form a main burner air and fuel mixture. The main burner mixture may be swirled by flow control surfaces in the main burners that impart circumferential motion to the axially moving main burner mixture. This swirled mixture continues downstream until the main burner flows and the pilot burner flow blend at which point the main burner flows are ignited by the pilot flame. The main burner mixture is usually leaner than the pilot burner mixture and hence stable combustion relies on the anchoring effect of the pilot burner mixture.
The premixing of the main burner flows is intended to reduce fuel consumption and emissions. Stability of the combustion flame in a premix combustor relies on proper premixing provided by the swirling effect of the swirlers in the main burners. Properly swirled and mixed flows reduce combustion instabilities and this, in turn, reduces lower NOx and CO emissions.
In conventional combustors the main burners are configured to impart swirl to each main burner flow in the same direction. When looking along a combustor axis, each main burner flow may be seen as rotating the same direction as the others. For example, each main burner flow may be rotating clockwise. However, in this arrangement, adjacent portions of adjacent flows travel in opposite directions. This creates shear and vortices that increase the heat release rate and emissions in the blending regions. To alleviate this it has been proposed to alternate the direction of the swirl imparted to the main burner flows such that they alternate between clockwise and counterclockwise. This is disclosed in U.S. Publication Number 20100071378 to Ryan, which is incorporated in its entirety herein.